ACEM Primary
Local anaesthetic pharmacology
- MOA = block Na+ channels in nerve axons
- Small diameter, myelinated first (type B, C fibers)
- High frequency depolarisation (type A delta)
- Sensory fibers on outside of nerve bundle
- Weak bases
- pKa = tendency to exist in charged/ uncharged form
- Low pKa ~ high % uncharged weak base
- Most pKa range 7.5-9.0
- At physiologic pH, majority of LA are charged cations-> active at R site
- Require uncharged form to penetrate cell membrane
- Infected tissue, LA less effective as low extracellular pH favors charged form
- Adding bicarb can increase uncharged form to enable penetration and reduce time to action
- Toxicity = sedation, visual/ auditory disturbances, circumoral or tongue numbness and metallic taste, nystagmus, muscle twitch, seizures or arrythmia (increased with bupivacaine)
| Esters Rapid plasma breakdown t ½ <1 min | Amides Systemic absorption reduced with vasoconstrictor Metabolised in liver |
| Cocaine Procaine Tetracaine Benzocaine | Lignocaine medium acting, t ½ 10 minduration 1-2 hr shortens APD (1B anti-arrhythmic) Mepivacaine medium acting, t ½ 7 min Bupivacaine long acting, t ½ 28 minduration 3-6 hr Ropivacaine long acting, t ½ 23 min duration 3-6 hr Articaine dental Prilocaine may induce methaemoglobinaemia |
Last Updated on August 12, 2021 by Andrew Crofton
Andrew Crofton
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